The present invention relates to a method of assembling a drivetrain for a wind turbine, and a wind turbine and an assembly kit thereof. First fixture elements are secured to first planet carrier of the first gear stage and second fixture elements are secured to the gearbox housing. The respective gear stages of the gearbox unit are pre-assembled, where the first planet carrier is fixed relative to the gearbox housing via the first and second fixture elements. The first ring gear is further fixed relative to the gearbox housing by fastener elements. The pre-assembled gearbox unit is then aligned with the main shaft and moved into position relative to each other. The gearbox housing is mounted to the main bearing housing, and the first planet carrier is released and moved axially into position relative to the flange of the main shaft.